Psoriasis is a systemic, immunological, genetic disease manifesting in the skin and/or joints. Because of its systemic nature, patients exhibit a broad spectrum of symptoms that vary in severity. Psoriasis is a lifelong, chronic, recurrent disease. In patient surveys conducted between 2001 and 2008 by the National Psoriasis Foundation, 33% of patients with mild disease and 60% of patients with moderate-to-severe reported that their disease was a significant problem in their everyday life. The severity score is a number that is used to classify the severity of psoriasis. A widely used severity score is the Psoriasis Area and Severity Index, or PASI score. The PASI score is calculated by dividing the body into a number of regions and grading the severity of the erythema (the red inflamed skin), and the severity of the scaling (the scaly, flaky skin typically found inside a lesion) within a region. That is, erythema or redness of skin is an important identifier for evaluation of the PASI score. The severity of erythema and scaling in PASI scores are estimated visually often leading to significant inter- and intra-individual variation in scores. Extra subjectiveness in the evaluation of erythema has been observed, since the perception of redness can be influenced by the skin tone, ambient lighting and many other such factors which are difficult to control in a clinical setting. Further, PAST scores require that the symptoms of several lesions are estimated which greatly increases the workload of dermatologists.
Computer aided methods for psoriasis severity scoring have been under investigation for a number of decades. In at least one technique, the severity scores for erythema are correlated with the hue (H) value and saturation (S) value in the HSV color model. The color differences between psoriasis lesions and normal skin were previously investigated, and the investigation concluded that the distribution of erythema severity is correlated with the difference in hue value. However, in that investigation, the color value is sampled randomly while ignoring the variation in lesion and skin color to assess psoriasis severity. In yet another technique, the colors of pigments in lesions are used to derive mean color values in RGB colors space and then these are used to grade the severity of lesions using K-Nearest Neighbors.